1. Field of the Invention
The present invention relates to a lithium secondary battery. In particular, the present invention relates to a lithium secondary battery in which a negative electrode active material containing silicon and oxygen is used.
2. Description of Related Art
Recently, with regard to a secondary battery having a high output and a high energy density, studies on lithium secondary batteries have been actively conducted. In lithium secondary batteries, charging and discharging are performed by causing lithium ions to move between a positive electrode and a negative electrode in a non-aqueous electrolyte solution.
Hitherto, graphite materials have been widely used as a negative electrode active material of lithium secondary batteries. However, when graphite materials are used as the negative electrode active material, it is difficult to sufficiently increase the capacity of the lithium secondary batteries. In view of this problem, studies on negative electrode active materials having a capacity higher than that of graphite materials have been actively conducted.
Examples of materials that have been proposed as negative electrode active materials alternative to graphite materials include silicon, germanium, and tin, all of which are materials that occlude lithium by forming an alloy with lithium. Among these, materials containing silicon are particularly considered to be promising materials because they have a particularly high theoretical capacity.
However, in such a negative electrode active material, e.g., silicon, which forms an alloy with lithium, the volume of the negative electrode active material changes with occlusion and release of lithium. Consequently, in a lithium secondary battery including such a negative electrode active material that forms an alloy with lithium, contact failure between a negative electrode active material layer and a negative electrode current collector tends to occur with the change in the volume of the negative electrode active material due to charging and discharging. Thus, a charge/discharge cycle lifetime may be shortened.
In view of the above problem, for example, Japanese Published Unexamined Patent Application No. 2004-349237 (Patent Document 1) discloses a negative electrode active material layer composed of a silicon oxide SiOx (0.5≦x<1) thin film formed by vacuum deposition or sputtering.
As described in Patent Document 1, by using a negative electrode active material containing oxygen together with silicon, a charge/discharge cycle lifetime can be extended, as compared with the case where a silicon-containing negative electrode active material layer that does not contain oxygen is used. However, when the negative electrode active material containing oxygen together with silicon is used, an irreversible capacity in the initial charging and discharging increases. Accordingly, an initial charge/discharge efficiency tends to be low.